Double-acting hydraulic cylinders have been used to actuate relatively movable parts of an aircraft. For example, double-acting hydraulic actuators are often utilized for deploying and retracting aircraft landing gear, leading edge slats, trailing edge flaps and other control surfaces.
Two particular criteria which must be satisfied for aircraft components, including hydraulic actuators, are high strength and light weight. Achieving these two goals in a single device is often difficult. That is, an increase in strength also typically results in a corresponding increase in weight. This has been a particularly acute problem for double-acting hydraulic cylinders which are used in high-stress areas. For example, landing gear hydraulic actuators often must withstand internal pressures in excess of 3,000 psi. In a typical double-acting hydraulic cylinder, this load is always experienced as stress by the various components.
A typical double-acting hydraulic cylinder comprises a hollow cylinder having a piston rod end and a cylinder head end. A piston rod having a piston attached at one end reciprocates in the cylinder. The other end of the piston rod is attached to a bearing which defines a transverse bore for pivotal attachment of the piston rod to an external structure. The piston rod end of the cylinder is sealed with a gland which permits the piston rod to reciprocate therethrough. The cylinder head has a supporting structure for a bearing which also defines a bore for pivotally connecting the cylinder head to an external portion of the aircraft. Hydraulic fluid is selectively introduced into the cylinder between the cylinder head and piston the extend the piston rod, or the gland and piston to retract the piston rod. Thus, as stated above, the cylinder and cylinder head, or cylinder and gland, are always under tension.
In addition to the tensional force described above, the cylinder head and gland are also subject to torques which result from friction between the bearings on the cylinder head or piston rod and the pivotally attached aircraft portions. The magnitudes of torque which the cylinder head experiences are typically larger than the torques encountered by the gland on the piston rod end of the cylinder.
Consider a landing gear system which is actuated by a double-acting hydraulic cylinder. The piston rod bearing may be connected, for example, to a strut on a landing gear, and the landing gear typically pivotally connected to the aircraft frame. The cylinder head bearing is typically connected to the air frame so that introduction of hydraulic fluid between the cylinder head and piston causes extension of the landing gear, and introduction of hydraulic fluid between the gland and piston retracts the landing gear. The torque generated by friction between the landing gear strut and piston rod bearing is transferred to the cylinder by the gland and the piston. Thus, torque is divided proportionately between these two structures. However, any torque generated at the cylinder head bearing is transferred to the cylinder through a connection between the cylinder head and cylinder only. Thus, a more robust connection has been employed between the cylinder head and cylinder (with a corresponding weight penalty) than between the gland and cylinder.
FIG. 1 illustrates a conventional prior art technique for joining a cylinder head 10 with a cylinder 12. As seen in the figure, the cylinder 12 has external threads 14 which mate with internal threads 16 on a downwardly extending lip 18 of the cylinder head 10. This mounting technique suffers from two distinct disadvantages. The internal and external threads have been known to fail under hydraulic pressures in excess of 3,000 psi. Furthermore, the cylinder head 10 is heavy and bulky.
FIG. 2 illustrates a second prior art technique for joining a cylinder head 20 with a cylinder head 22. In this prior art technique, the cylinder has a peripheral, radially extending flange 24 which mates with a corresponding flange 26 on the cylinder head 20. Bolts 28 connect the flanges. The structure shown in FIG. 2 is somewhat stronger than the structure shown in FIG. 1. However, the additional weight and bulk of the flanges and bolts are undesirable. Thus, a need exists for a lightweight structure having sufficient strength to join a cylinder head with a cylinder and hydraulic fluid actuator.
It is known that a gland 30, shown in the righthand side of FIG. 3, can be secured to a cylinder 32 by means of a segmented shear ring 34. The shear ring has multiple arcuate segments which transmit axial force from hydraulic pressure in the cylinder to a radially outward force against the cylinder. This mounting structure is low in weight and provides a high-strength connection between the gland and the cylinder. However, it is believed that this technique would be inapplicable to mounting the cylinder head to the cylinder because of the large, torqueinduced stresses encountered by the cylinder head.